What are inclusion and exclusion criteria?

Inclusion and exclusion criteria define the scope of the search protocol.

The criteria are tied directly to the research question, and are generally categorized around the research framework sections (e.g. PICO- population/platform, interventions, comparators, outcomes.)

In these statements, all terminology and the extent of included concepts should be clearly defined.

The statements should be so clear in meaning and intent that any person, either internal or external to the project, should be able to categorize an information source as to whether it will be included in the synthesis or not.

This aids in ensuring that multiple investigators will categorize resources in the same way (inter-rater reliability), as well as improve reproducibility if another researcher chooses to use the protocol again.

Creating Inclusion Criteria

 The elements of the research question describing the context of the research question (in the case of PICO, population, intervention and comparator) are described rigorously and thoroughly for the purpose of defining which studies are eligible for the final data set. Outcome or the resulting element of the research statement is not generally described in inclusion and exclusion criteria because the answer to the research question is generally a range of outcomes that could be unpredictable and varied.

For studies that are describing a specific population, there may be multiple inclusion criteria. It may include gender, race, age, height, sex or any other clearly descriptive criteria that matches the research question. This may also include geopolitical boundaries, personal social, health, or economic factors, or any other aspects that define the population as a subset.

In cases where specific data is being identified, standardized measurements, validity ranges (variable x cannot be below 20 Hz  or above 50 Hz), and other known quantitative measures that should appear in the article are listed as well.

In projects that are extensive in scope, the variables may be defined in a separate data dictionary for the purpose of clearly articulating every related term so that any individual in the project can refer to the list while using the criteria.

Criteria are specific to the project and research question. Criteria impact on research methodology decisions and should be recorded in the protocol and methodology section of any resulting publication.

Creating Exclusion criteria

While inclusion criteria are frequently focused on the research question, the exclusion criteria are frequently focused on limiting the search set based on characteristics of the publication. Common exclusion criteria include:

• Restricting to one specific language
• Imposing a date range (Earliest date; latest date; or between two given dates)
• Restricting articles by research methodology, population or other details.

Exclusion criteria should seek to further clarify inclusion criteria. While inclusion criteria may state that adults are included, the exclusion criteria may clarify that all articles that are focused on in vitro and animal studies are excluded.  This further clarifies the scope of the inclusion criteria above, and therefore enhances the methodology. Exclusion criteria that simply mirror the inclusion criteria are not necessarily helpful. For instance, an inclusion criteria may say "All studies must be conducted in the state of Illinois". Having an exclusion criteria that states "All studies conducted in any state but Illinois" doesn't further clarify the protocol and is not needed. On the other hand, the inclusion criteria "All studies use grounded theory" is augmented by the exclusion criteria, "Mixed-methods studies are excluded".  In this case, the criteria add clarity in that while some mixed-methods studies use grounded theory, they are excluded based on the presence of additional methodology. 

For the following linked articles, identify the inclusion and exclusion criteria for the study.  Based on the research question, why were these selected? If you were creating this project, are there any criteria that you would have added to the list?

Conte et al. 2023. Complications of Body Piercings: A Systematic Review. Cutis 112(3): 139-145. https://doi.org/10.12788/cutis.0847

Dankiw et al. 2020. The impacts of unstructured nature play on health in early childhood development: A systematic review. PLOSOne. https://doi.org/10.1371/journal.pone.0229006